I started playing dota in 2019. I guess by 2020 you must have already experienced the 5 couriers update. When I started playing dota, Couriers cost money, and there could be just one cour per team. now there are 5. and once killed, they respawn after a while.

Shrines used to heal allies in the triangle/jungle. no more shrines. Now there is one convertible outpost in the triangle, and one nonconvertible outpost at the extreme end of the safe lane.
techies has been reworked. atleast 3 times. cant explain. wont explain.
there is a skyscraper meepo. cant explain. wont explain.
Aghanim scepter can be upgraded to blessing to free the slot.
killing neutral creeps gives neutral items.

wisdom rune. spawns every 7 minutes. gives xp. No more tome of knowledge.

stout shield is gone. Enter blood granade.

many items recipies have been changed. e.g. rod of atos. Also, bkb seems to be pretty heavily nerfed in 2-3 years. bkb vs maledict. bkb vs mana drain. bkb is just sad. Rod of atos has an upgrade now. Eul’s scepter has an upgrade now. Too many changes to recount.

zeus is a usually played as a carry hero now. in herald-legend bracket. Aah yes. now there is no real difference between herald and legend. more griefers and feeders in herald. thats it.

jakiro is playable as a hard sup or as a mid semi-carry. watch dota channels on youtube for jukes.

Obs wards cost 0. sentries cost 50. If i remember, obs cost was 50 and sentry was 100, in 2019.

blink dagger has 3 possible upgrades. str/agi/int.

bloodstone reworked. bb + bloodstone juke. Also bb reworked. i think. I dont remember bb from 2020.

Aghanim’s shard. Like aghanim’s scepter, enhances an ability or gives a new ability. Unlike scpeter, it doesnt give stats or takeup slots. Good for many heroes including wd, medu.

Ogre Magi is now a core hero. pos1,2,3. OM has 0 intelligence. gains mana and mana regen from str, and items. right click triggers fireblast with a small probability. so.

new heroes added. snapfire. muerta. primal beast. marci. void spirit. maybe i missed 1 or 2 heroes. but they all pretty much have high mobility, either escape or initiate.

Map is 20% larger in length approx. so walking takes more time. And, more space also means more creep camps. so more gold to farm. and almost every hero can scale to late game. even hard supports. cm,wd,jakiro, rubik,sky,silencer,lion,shadow shaman,shadow demon, etc can all be played as soft/hard sup and after around 30-40 minutes, can start building 4k-5k gold items. Imagine sky with aeon disk, sange&kaya,octarine,strength blink, lotus. If enemy carry had a bad game, sky can actually survive 1v1 against enemy carry for 5-10 seconds.

In summary,… play a few unranked. Too much difference between then and now.